1. Field of the Invention
This invention is concerned with the conversion of dialkyl disulfides to hydrogen sulfide and useful hydrocarbons. More particularly, it is concerned with the disposal of dialkyl disulfide waste streams such as those produced in caustic extraction of mercaptans from gasoline or other liquid hydrocarbon products.
2. Prior Art
An unacceptable concentration of sulfur as aliphatic mercaptans is often encountered in the refining of liquid hydrocarbons to make gasoline, kerosine, jet fuel or heating oil, for example. These mercaptans, mostly in the C.sub.1 -C.sub.4 range, are objectionable for a variety of reasons. In addition to imparting an unpleasant odor to the product, mercaptans attack copper alloys with the formation, in some instances, of gels or deposits; in all but the smallest quantity, mercaptans react with tetraethyl lead and reduce its anti-knock effectiveness. In all instances, sulfur compounds are converted to corrosive sulfur oxides on combustion, and emissions of these oxides deteriorate the quality of the air.
When the total sulfur content of the hydrocarbon product might be acceptable were it not for the mercaptan content, the refining industry sometimes resorts to non-extractive "sweetening" by oxidizing the mercaptans to dialkyl disulfides according to the general equation EQU R.sub.1 SH+R.sub.2 SH+Oxidant.fwdarw.R.sub.1 SSR.sub.2 (1)
wherein R.sub.1 and R.sub.2 individually are alkyl groups that contain one or more carbon atoms. In such instances a variety of oxidants may be used including, by way of illustration, sodium plumbite and sulfur ("Doctor" process), or air in the presence of a catalyst (Merox sweetening). In all such instances the total sulfur content of the product is not substantially changed, the disulfides remaining in the product.
However, when the total sulfur content of a hydrocarbon stream is too high to produce an acceptable product, it becomes imperative to resort to removal of sulfur. Catalytic hydrotreating may be used for this purpose, and although highly effective it consumes costly hydrogen and converts the mercaptans to low molecular weight hydrocarbons of relatively low market value. In its favor, however, it is noted that catalytic hydrotreating converts the organic sulfur to hydrogen sulfide which is conveniently processed in a Claus unit to convert it to marketable sulfur. Thus, catalytic hydrotreating does not create a difficult waste disposal problem.
Another alternative available to the refiner is to extract aliphatic mercaptans with caustic soda. The mercaptans which are most objectionable, i.e. those in the C.sub.1 -C.sub.4 range, fortuitously are the very ones which are most soluble in a caustic wash. A variety of caustic wash processes are available, most of them depending on the addition to the wash of one or more organic materials, sometimes referred to as "solutizers", which have been found to increase the solubility of the mercaptans. Such additives include cresoles and naphthenic acids, by way of illustration. The term "caustic wash", unless otherwise explicitly stated, will be used herein to refer broadly to any aqueous sodium hydroxide or potassium hydroxide solution of a concentration suitable for mercaptan extraction whether or not it contains, e.g., potassium cresylate. The various caustic wash processes are described in detail in a book by Dr. R. N. Maddox, "Gas and Liquid Sweetening", published by John N. Campbell, Norman, Okla. 73069, Library of Congress Catalog Card No. 73-91966. In particular, pages 202 to 220 of that volume are incorporated herein by reference for general background on the state of the art of liquid sweetening.
In the caustic wash process, the aqueous caustic solution, regardless of the presence or absence of "solutizers", becomes saturated with mercaptans. It may be regenerated by reboiling in the range of 220.degree.-240.degree. F. to strip the mercaptans. In a preferred operation, however, the mercaptans are oxidized, for example, by blowing with air in the absence or in the presence of a catalyst (e.g. Merox extraction). The disulfides formed on oxidation are usually insoluble in the caustic wash and may be decanted. The presence of potassium cresylate, however, tends to solubilize the disulfides. In such instances, the separation of the disulfides is facilitated, or may even require, extraction by a naphtha such as a straight-run naphtha boiling at 300.degree.-400.degree. F. Thus, for purposes of the present invention, caustic wash processes in which a naphtha extraction is used to assist in the separation of the aliphatic disulfides are contemplated along with other variants. For extraction by naphtha, one volume of the caustic wash may be contacted with about 0.1 to 5 volumes of the naphtha.
In general, a caustic wash process in which the caustic is regenerated by oxidation to the disulfides results in the production of a liquid waste stream consisting of one or more dialkyl disulfides that is difficult to dispose of. This waste stream is usually passed to a catalytic hydrodesulfurization unit where the disulfides are converted to hydrogen sulfide and alkanes having one to about four carbon atoms that are of relatively low market value. Although otherwise effective, the disposal of the waste stream places the process as a whole in a disadvantageous position compared with other alternatives because of the heavy burden placed on the catalytic hydrodesulfurization unit in handling a stream with a very high sulfur content.
It is an object of this invention to provide a novel method for disposing of a liquid stream consisting of a mixture of aliphatic dialkyl disulfides wherein each alkyl group has from one to about four carbon atoms. It is a further object of this invention to provide a method for converting such aliphatic dialkyl disulfides to hydrogen sulfide and hydrocarbon liquids. It is a further object of this invention to provide an improved caustic wash process for reducing the mercaptan content of a liquid hydrocarbon stream, the improvement consisting of an improved catalytic method for disposing of the disulfide waste stream. These and further objects will become apparent to one skilled in the art on reading this entire specification including the claims thereof.
A fairly large number of patents have appeared which are concerned with the conversion of aliphatic hetero atom containing compounds such as methanol and dimethyl ether to higher hydrocarbons. These patents generally utilize a crystalline zeolite such as ZSM-5 as catalyst. U.S. Pat. No. 3,894,107 issued July 8, 1975 to Butter et al. describes the conversion of aliphatic alcohols, aliphatic mercaptans, aliphatic sulfides and aliphatic halides to hydrocarbons. Example 25 thereof illustrates the conversion of methyl mercaptan. U.S. Pat. No. 3,894,106, issued on July 8, 1975 to Chang et al describes the conversion of aliphatic ethers, such as dimethyl ether, to aromatic hydrocarbons. U.S. Pat. No. 3,728,408 issued Apr. 17, 1973 to Tobias describes the conversion of polar organic compounds by catalytic contact with a crystalline aluminosilicate catalyst, the silica/alumina ratio of which is more than 10/l. Benzyldisulfide is noted therein (Col. 7, line 1) along with numerous other sulfur-containing and sulfur-free compounds. ZSM-5 is included in the recitation of catalysts. The Examiner's attention is called also to a publication by C. D. Chang and A. J. Silvestri in the Journal of Catalysis, Vol. 47, pp. 247-259 (1977), particularly to Table 1 wherein the conversion of methyl mercaptan in the presence of ZSM-5 catalyst is reported.
The above references, although they do not suggest the present invention, are brought to the attention of the Examiner should he wish to consider them in his examination for patentability of the present invention. Applicants do not know of any other reference which, in their opinion, discloses or might be considered to suggest the present invention.